When you create a professional voicemail greeting, it can be a good idea to think about what your callers might want to know. After doing so, you can make a list of points to cover in your voice message; it might also be a good idea to write out the entire greeting on a piece of paper. The ideal message should be somewhere between 10 and 20 seconds, so you may want to time yourself repeating your message before you record it. After composing your telephone message, it can be a good idea to get a second opinion of this greeting so you can make sure it sounds professional. Writing down a voicemail greeting helps prevent mistakes when recording it.
Context does matter here. If the caller would reasonably expect you to answer the phone (such as if they were calling a store or other place of business), an apology for not being able to get to the phone makes the most sense.

Start Skype and choose Tools→Options→Calls→Voicemail from the main menu.Put a checkmark in front of Send unanswered calls to voicemail.Just click the Record button (the green button with the triangle) and speak the Voicemail greeting you want callers to hear.Click the Stop button (the red button with the dot) when you're finished recording.Click the Replay button (the red button with the backward arrow) to listen to the message you just recorded.When you're satisfied that you recorded the perfect message, click Save.See More....
Another great information you can put in your voicemail messages is to inform your prospects when you can reach out to them say the next morning or mid-afternoon and more so that they can be aware of when they can expect a call from you.